1. Field of the Invention
The present invention relates to hydrocarbon separation, and more particularly to extraction, including liquid-liquid solvent extraction units and extractive distillation units.
2. Description of the Related Art
Aromatic hydrocarbons, such as benzene, toluene and xylenes (collectively, “BTX”), serve as important building blocks for a variety of plastics, foams and fibers. Often these compounds are produced via catalytic reformation of naphtha through steam cracking of naphtha or gas oils, or other methods where substantial amounts of non-aromatic compounds are present. When simple distillation or fractionation is not a cost effective or practical method for separation, liquid-liquid extraction or extractive distillation techniques are used. Such extraction techniques separate a desired substance selectively from a mixture or remove unwanted impurities from solution, and, in the context of the invention described hereinbelow, aromatic hydrocarbon separation from non-aromatic hydrocarbons. A solvent is typically used which exhibits a higher affinity for the aromatic compounds than the non-aromatic compounds, thereby selectively extracting the aromatic compounds from the mixture of aromatics and non-aromatics. The aromatic species of interest can then be isolated from the solvent by distillation, adsorptive separation techniques, and the like.
One widely used solvent extraction technique is the Sulfolane™ Process to recover high purity aromatics from hydrocarbon mixtures, which is discussed in numerous patents and other literature too numerous to cite. The process uses a combination of liquid-liquid extraction and extractive distillation in a single, integrated design, and employs tetrahydrothiophene-1,1-dioxide (or “sulfolane”) as a solvent and water as a co-solvent. Other solvents conventionally used in liquid-liquid extraction include oxygen-containing species such as tetraethylene glycol and nitrogen-containing species such as N-methyl pyrolidine, each having similar issues that lead to decreasing the capacity of the extraction process, as well as their own unique problems.
FIG. 1 depicts a schematic view of an illustrative system 400 for separating high purity aromatics from a hydrocarbon mixture. Particularly desired products include benzene, toluene, xylene, and mixtures thereof (collectively, “BTX”), which are typically obtained in a desired purity (“on-spec”) by further distillation or other methods downstream of the illustrative system 400. Conveniently this benzene and or toluene product fractionation section may be fully or partially heat integrated to provide feed preheating and or reboiling duty to the benzene and or toluene columns with other equipment in the installed facilities outside of System 400. Various embodiments are possible to make the heat integration feasible in order to create sufficient temperature differential between the heat supply source and the benzene and toluene columns that are receiving the heat. Examples include but are not limited to: raising the pressure in the column that is supplying the heat or lowering the pressure which might include operating under vacuum conditions in the benzene or toluene columns that are receiving the heat. In addition to one or more liquid-liquid extractor unit(s) 100 described above, the system 400 includes one or more extractive distillation units 410, water strippers 420, water wash columns 430, and recovery columns 440. In one or more embodiments, the system 400 can further include one or more heat exchangers (three are shown 445, 450, 455), one or more steam generators 460, and one or more water/hydrocarbon separators (two are shown 465, 470). Certain other devices, such as compressors, valves, column trays and packings, and the like, are not shown for convenience of view, however such devices would be apparent to one of ordinary skill in the art.
Within the extractor 100, a hydrocarbon mixture via line 145 (entering the extractor 100 at one or more locations) and a circulating solvent via line 150 can be contacted or otherwise mixed with one another in a countercurrent manner. The internal details of the various apparatus such as liquid-liquid extractor 100, extractive distillation unit 410, stripper 420, and column 440 do not form a part of the present invention per se except as explicitly pointed out herein below, but rather have been described in the prior art such as in U.S. Pat. No. 7,288,184, U.S. Patent Application Publication 2010-0096321, and more recently U.S. Provisional Application No. 61/566,116, filed Dec. 2, 2011, and may consist of various trays, gratings, packings, demister pads, and the like. Within the liquid-liquid extractor 100 the solvent extracts or otherwise separates at least a portion of the aromatic hydrocarbons from the multi-component hydrocarbon introduced via line 145 to provide a solvent enriched in aromatic hydrocarbons (“rich solvent”) via line 155 and a raffinate having a reduced content of aromatic hydrocarbons via line 160. The solvent is advantageously Sulfolane™ but numerous other suitable materials for separating aromatic compounds from non-aromatic compounds are known in the art. The hydrocarbon feed via line 145 can be or include a mixture of aromatics and non-aromatics, typically in the C5-C10 range, and may also be or include a heavier refinery cut.
The rich solvent via line 155 is introduced to the heat exchanger 445 to transfer heat from the lean solvent introduced via line 462 to provide a heated rich solvent via line 447 and a cooled lean solvent via line 150. The heated rich solvent via line 447 can be introduced to the stripper 410 to provide a less-aromatic rich hydrocarbon via line 412 and a solvent further enriched in aromatic hydrocarbons via line 414.
The raffinate in line 160 can be introduced to the raffinate wash column 430 which can separate at least a portion of the solvent in the raffinate to provide a raffinate product via line 432 containing less solvent than the raffinate in line 160. The recovered water/solvent in line 434 can contain aromatics/non-aromatics separated in and/or entrained from the raffinate wash column 430 from the raffinate introduced via line 160. The recovered water/solvent in line 434 can be introduced to the water stripper 420 to provide a water-lean, hydrocarbon-rich stream via line 422. The non-aromatic rich raffinate via line 432 can be further processed or sent to storage.
The water-lean, hydrocarbon-rich stream in line 422 can be introduced to the water/hydrocarbon separator 470 to provide a recycle hydrocarbon via line 472 and a recovered water stream via line 474. In one or more embodiments, the recycle hydrocarbons via line 472 can be introduced at one or more locations to the extractor 100 for additional processing and/or mixed with the feed line 145 (not shown for convenience of view).
Within the recovery column 440, the bottoms from the stripper 410 is contacted with steam to recover the aromatics. The aromatic compounds are removed from the top of the recovery column 440 and the bottom stream (lean solvent) 462 is recycled back to the extractor 100, although a portion may be sent to solvent regenerator 442 and introduced to recovery column 440 via line 443. The overhead from the recovery column 440 is introduced to the water/hydrocarbon separator 465 to separate the water via line 467 from the product aromatics via line 466. At least a portion of the recovered aromatics can be recycled to the recovery column 440 as reflux.
The process described above, as a whole, is known in the art and further detail here is not necessary to inform the art. See also U.S. Pat. Nos. 7,326,823; 2,773,918; and 3,361,664.
Nevertheless the operation of an aromatic solvent extraction system to form an aromatic extract is exceedingly complicated to operate. Part of the reason for this is that, relative to each aromatic product recovered from the process, the lighter and heavier non-aromatics in a given feed respond differently to changes in process parameters such as the flow rates of the feed and solvent. Thus, a single change in one such parameter can cause widely varying results in the process and products thereof U.S. Pat. No. 7,326,823 solves attempts to control the system by analyzing at least two separate groups of non-aromatics, and, according to the patentee, thereby knowing the relative concentrations of both the lighter and heavier non-aromatics, as opposed to the prior art's single total concentration of non-aromatics, and again according to patentee, the proper adjustments to operating parameters of the process can be made to allow, pursuant to the invention, for tighter control of the final aromatic product(s) purity.
In addition, there continues to be the problem of light impurities building up in the extractive distillation tower and recycle system. These undesired effects result in the incapacity of the extractor to efficiently remove and recover the aromatic compounds within the mixed feedstock.
Typical responses to correcting the incapacity of the extractor include one or more of moving the recycle location, adding more stages of sieved trays, reducing operating rates, or cleaning and/or replacing the sieve tray decks.
The present inventors have discovered that proper monitoring and control of the extraction unit systems in the separation of aromatic hydrocarbons from non-aromatic hydrocarbons, including liquid-liquid extraction processes and extractive distillation processes, and the combination thereof, can result, in embodiments, in at least an order of magnitude improvement in reliability and integrity of the solvent systems and processes for the separation of aromatic hydrocarbons and non-aromatic hydrocarbons.
In addition, giving the operators information about how to respond to certain situations has also proven beneficial. In U.S. Pat. No. 7,739,217 there is disclosed a method for monitoring a polyethylene polymerization system including providing an expert system comprising a database containing knowledge of the polymerization system and an inference engine, the latter comprising rules, receiving and evaluating data from the polymerization system, identifying the first rule as true or false, and displaying the message of the second rule if all the one or more preconditions of the second rule are met. The present inventors have also provided an improved expert system and inference engine wherein the improvement comprises application in extractive distillation controls. As used herein further below, the terms “expert systems” and “inference engine” take the same meaning as set forth in U.S. Pat. No. 7,739,217.